The invention relates to an ion implantation device which includes: an ion source for producing a beam of ions to be implanted into a substrate, an acceleration electrode for accelerating the ion beam emanating from the ion source, ion-optical elements which are arranged downstream from the acceleration electrode in order to influence the direction of the ions in the ion beam, and a deceleration device which is arranged downstream from said ion-optical elements in order to decelerate the ion beam.
The invention also relates to a method of implanting ions in a substrate.
A device of the kind set forth is known from the abstract in English of Japanese patent application No. 3-47123, filed on Mar. 13, 1991 and published under publication No. 4-284343 on Oct. 8, 1992.
Ion implantation is commonly used in the manufacture of integrated circuits in order to form specified doping profiles, for example a specified doped ion concentration as a function of the depth in the substrate. The ion beam required for this purpose is produced in known manner by an ion source, after which the beam is accelerated to a desired velocity by an (electrostatic) acceleration electrode which directly succeeds the ion source. For the further influencing of the ion beam such a device may also be provided with ion-optical elements, such as a deflection device for scanning the beam across the substrate to be doped and charged particle lenses for focusing or otherwise converging or diverging the ion beam.
Subsequent to the acceleration electrode said known device is provided with an ion-optical element in the form of a mass separation unit for separating ions having an undesirable mass from the ion beam, so that the ion beam thus produced consists of one type of ion only. From the particle-optical technique it is generally known that in order to achieve suitable and controlled influencing of the ion beam by the ion-optical elements is desirable that the ion beam has a sufficiently high velocity, for example a velocity which corresponds to a kinetic energy of the order of magnitude of from tens to hundreds of keV. A typical value in this respect is 30 keV, thus corresponding to a voltage of 30 kV on the acceleration electrode (the acceleration voltage). This is because when the energy of the bam is too low (for example, 1 keV), the beam becomes highly susceptible to disturbing influences from inside and outside the apparatus and to undesirable expansion of the beam due to space charging in the beam.
Said specified doping profiles often require the ions to be implanted only in a zone up to a specified depth in the substrate to be doped. To this end, the ions may be incident on the substrate only with a given, specified velocity, i.e. energy. This specified energy may typically be of the order of magnitude of 1 keV. In order to conduct the ion beam through the ion-optical elements with a sufficiently high energy and nevertheless make the beam land on the substrate with the specified energy, downstream from said ion-optical elements there is arranged in known manner a deceleration device for decelerating the ion beam to the desired energy.
The described processes take place in an evacuated space. The vacuum of this space is often of poor quality because gases are released during irradiation of the substrate by means of ions (notably from the residual material on the substrate), which ions are spread through the vacuum space. During traveling of the path from the ion source to the deceleration device, interaction with the released gases and the residual gases always present in the apparatus neutralizes a part of the ions in the beam. These neutralized ions (i.e. atoms) are no longer sensitive to influencing by the ion optical elements and the deceleration device, so that these atoms strike the substrate with the full energy of, for example 30 keV and hence penetrate therein to a depth which is much greater than the depth corresponding to the specified doping profile. Moreover, such atoms are not sensitive to fields applied for scanning the beam across the substrate to be treated, so that these atoms form a stationary dot "spot" at the center of the substrate region to be doped, thus locally causing an inadmissibly high concentration of the relevant element in the substrate. In order to counteract the problems concerning the neutralized ions, the deceleration device for decelerating the ion beam in the known ions implantation device is also arranged to deflect the ion beam. The neutralized ions (i.e. the atoms) which are not sensitive to electromagnetic deflection then continue their travel in the original direction and hence can be separated from the deflected ion beam.
In these known devices a problem is encountered in that the deceleration device consists of an assembly of three electrodes which together constitute an electrostatic lens. The first electrode of this lens carries a potential which amounts to a fraction of the acceleration voltage (thus, this first electrode is actually formed by the boundary of the drift space carrying said potential); the third electrode of this lens carries ground potential (the third electrode is actually formed by the entrance of the treatment space of the substrate which carries ground potential), whereas the central electrode carries a potential which lies between said two potentials. In particle optics it is generally known that electrostatic deceleration is ineviatably accompanied by a lens effect exerted by the decelerating field. Due to this lens effect, the ion beam is subjected to a diverging or a converging action. For said order of magnitude of the acceleration voltage and the ultimate speed of landing of the ion beam, the ions in the beam are given an inadmissibly large velocity component transversely of the beam axis due to said diverging or converging effect. Consequently, a significant part of the ions would not reach the substrate, because they would be intercepted by beam limiters between the deceleration electrodes and the substrate. Moreover, a large angular spread of the ions in the beam could cause a shading effect on the substrate to be doped. This means that the ion beam which apparently emanates from one point fans out in a conical manner, so that regions on the substrate which directly adjoin an edge of a region with a given difference in height with respect to the remainder of the substrate are situated in the shade of said edge and hence receive fewer ions than the regions which are not situated in the shade. For these two reasons the angular spread of the ion beam incident on the substrate must be small.